1. Field of the Invention
The present invention relates to a power amplifier and a wireless communication device capable of limiting a degradation in a distortion characteristic which occurs with a change in load.
2. Background Art
GaAs power amplifiers are presently used widely as a power amplifier for portable telephones including those used in CDMA systems. FIG. 10 is a block diagram showing a wireless communication device for a portable telephone for use in a CDMA system.
In FIG. 10, ANT denotes an antenna terminal through which a signal is transmitted and received. BB denotes a baseband signal processing section which demodulates and decodes a received signal, and which encodes and modulates a signal to be transmitted. RF-IC denotes an RF circuit which performs predetermined frequency conversion on the signal output from the section BB. BPF denotes a bandpass filter. PA denotes a power amplifier which amplifies a signal output from the section RF-IC. ISO denotes an isolator which is a unidirectional power transmitting device for eliminating the influence of variation in load at the antenna terminal ANT on the power amplifier PA. DUP denotes a duplexer which is a bandpass filter through which a signal to be transmitted is supplied from the power amplifier PA to the antenna terminal ANT, and through which a received signal from the antenna terminal ANT (in a band different from that for the transmission signal) is supplied to the receiving section RF-IC.
FIG. 11 is a circuit diagram showing a conventional power amplifier. Circuit elements surrounded by the broken line in FIG. 11 are formed on a GaAs chip. Other circuit elements are formed as chip components and lines on a module circuit board.
IN denotes an input terminal through which an RF signal is input. OUT denotes an output terminal through which an RF signal is output. Transistors Tr1 and Tr2 are heterojunction bipolar transistors (hereinafter referred to as “HBT”) for amplifying power. Transistors Tr1 and Tr2 are provided in an initial stage and in a final stage, respectively. Bias1 and Bias2 denote bias circuits for supplying bias currents to the bases of the initial-stage and final-stage transistors Tr1 and Tr2. Vcb denotes a terminal through which power is supplied to the bias circuits Bias1 and Bias2. Vref denotes a terminal through which a voltage is input to the bias circuits Bias1 and Bias2.
Rb1, Rb2, Rb12 and Rb 22 denote resistors, C1 to C4, C21 to C23, Cd1, Cd2 and Cdb, capacitors; L1 and L2 inductors, L11 and L21 to L23, lines having particular electrical lengths and functioning as inductors, and Vc2 and Vb2, collector and base voltages on the transistor Tr2.
FIG. 12 is a circuit diagram showing the bias circuits Bias1 and Bias2. Trb1 to Trb5 denote HBTs, and Rbb1 to Rbb5 resistors. Vrefb denotes an input voltage terminal for the bias circuits Bias1 and Bias2, and Vcb a terminal for power supply terminal to the collectors of the bias circuits Bias1 and Bias2. Vbo1 and Vbo2 denote output terminals of the bias circuits Bias1 and Bias2. The bias circuits Bias1 and Bias2 operate so that idling current through the power amplifying transistors Tr1 and Tr2 (bias currents when no RF signal is input) are constantly maintained with respect to temperature (see, for example, Japanese Patent Laid-Open No. 2004-343244).
A distortion characteristic of a power amplifier largely influences variation in load at an antenna. In CDMA power amplifiers or the like in which particular importance is attached to a distortion characteristic among output characteristics, therefore, an isolator ISO is provided immediately after the power amplifier PA as shown in FIG. 10 to prevent the power amplifier PA from being influenced by variation in load at the antenna ANT. However, the isolator is comparatively high-priced among portable telephone components and substantially large in height. There is, therefore, a strong demand for a power amplifier designed to be reduced in price and size while ensuring the desired distortion characteristic without using the isolator. The adjacent channel leakage power ratio (ACLR) will be considered as an index of the distortion characteristic in the following description.
FIG. 13 shows a circuit for a load variation test on a conventional power amplifier without any isolator. In this circuit, a tuner for changing the impedance of the load on the circuit shown in FIG. 11 is provided on the circuit shown in FIG. 11. When the impedance ZL of the tuner is changed, the output load is changed.
FIG. 14 is a diagram showing characteristics of the power amplifier shown in FIG. 13 with respect to the phase of the tuner (output load). The ACLR, the power gain, collector current Ic2 in Tr2, collector voltage Vc2 and base voltage Vb2 of Tr2 and bias current Icb2 in Tr2 when forward wave output power as monitored through a coupler was constantly maintained were measured. The input power changes with respect to the phase by following the change in gain.
Curves CA, CB, and CC in FIG. 15 are examples of load curves with respect to an intermediate point A, a high-current point B, and a low-current point C in the collector current (operating current) Ic2 shown in FIG. 14. The actual load curves are expanded as shown in FIG. 16 due to a large reactance component. However, a description is made here by using FIG. 14 for simplification of interpretation.
At the high-current point B, the load curve has a steep gradient as indicated by CB in FIG. 15, and the increase in collector current Ic2 is large. At the low-current point C, the load curve has a gentle gradient as indicated by CC in FIG. 15, and the increase in collector current Ic2 at an idling point A1, which is a bias point when no RF input power is supplied, is small. Conversely, the gain is low at the point B but high at the point C.
The distortion characteristic (ACLR) of the power amplifier is determined by distortion balance between the initial-stage and final-stage transistors. Therefore, when the collector current Ic2 is low as at the point C, a degradation in ACLR recognized by comparison when the output power is fixed is large and there is a possibility of failure to meet the requirement of ACLR. These characteristics are summarized in FIG. 17.
Power amplifiers in which the bias for a final transistor is changed by detecting input power and output power have also been proposed (see, for example, Japanese Patent Laid-Open Nos. 2001-257540, 7-22857, 2004-72250, 2002-43855, and 11-220338).
In a CDMA power amplifier, however, output power is adjusted while controlling input power and the input and output powers change at all times according to the distance between the terminal using the amplifier and a base station. Therefore, a low-operating-current condition also exists in low-output operation and it cannot be discriminated form the low-operating-current condition at the point C. In the case of a low output, the power supply voltage is reduced by using a DC-DC converter to reduce the consumption of power. In such a case, the power gain is also reduced and it is difficult in practice to detect a change in load by detecting the power gain. Further, the voltage value of Vb2 increases when the output is low, and the low-output condition cannot be discriminated from the condition at the point C through detection of Vb2. For this reason, a change in load cannot be detected by detecting the input power and output power in the conventional art. Thus, there is a problem with the conventional power amplifier in that the distortion characteristic degrades when the load changes.